I'm trying to take a copy of a plastic part I have which is slightly the wrong shape/angle - my plan being to modify the original, then create a better version while keeping the old one as a backup for when I mess it up!

The original has a textured surface (your standard grippy plastic surface 0.5mm patterning). I'd like to temporarily and non-destructively fill that, so I can take a multi-part mould from it. I'm obviously concerned about releasing the pattern from the mould afterward - what technique would you use?

Better to just take a quick splash mould of it as is, then use that mould to produce a fibreglass copy that you can modify and paint and bring up to the finish you want.

The splash mould can just be 3-4 layers of csm and regular polyester or vinyl ester laminating resin, as cheap as you like.